1. Field of the Invention
This invention relates to a copper clad laminate useful for, e.g., production of printed-wiring boards, more particularly, to a copper clad laminate with reduced warping and torsion generated during processing.
2. Description of the Prior Art
A copper clad laminate has been produced by pressing a copper foil as an inorganic material and belonging to a metallic material and an organic material as an insulation layer constituent, represented by prepreg, to adhere them to each other under heating. Therefore, a copper clad laminate is composed of copper foil/insulation layer constituent material/copper foil as a basic structure, and is normally produced by hot-pressing the layered structure at around 180° C.
The hot-pressed copper clad laminate has various problems. One of the problems is a phenomenon known as warping (hereinafter simply referred to as “warping”) of the laminate itself, observed when it is taken out of the press on completion of hot-pressing, cooling and disassembling the built-up pressed body. Another problem is a phenomenon known as torsion (hereinafter simply referred to as “torsion”) of the copper clad laminate as a whole. This phenomenon is considered to be one type of the warping phenoma.
The warped or distorted copper clad laminate should cause various problems, when used for production of printed-wiring boards without being corrected, in all of the steps of surface finishing, registration and etching of an etching process. The resultant circuit with copper foil cannot keep required precision, making it almost impossible to form a fine-pitch circuit.
The persons skilled in the art have been trying to correct a warped or distorted copper clad laminate by adding a post-treatment step, such as heat treatment referred to as after-baking or keeping the laminate under load for an extended period to make it flat.
Addition of such a post-treatment step to correct the warping or torsion of the copper clad laminate leads to increased production cost. Cost increase should be avoided as far as possible for Japanese manufacturers to keep their competitiveness in the world market.
The manufacturers have been taking various countermeasures to prevent warping and torsion, considering that these problems are caused by strains generated within the laminate, resulting from difference between the constituent layers in thermal expansion behavior during a hot-pressing process and in contraction behavior during a cooling process. These countermeasures include (1) changing material for a mirror plate used during the hot-pressing process to the one having a coefficient of thermal expansion closer to that of copper foil, (2) controlling the mirror plate surface roughness in such a way to minimize the effects of thermal expansion or contraction behavior of the mirror plate on a deformation behavior of the copper foil, (3) changing a glass-epoxy base, commonly referred to as a prepreg, used as an insulation layer constituent; for example, changing glass material for a glass cloth, shape of glass fibers, or modification of the epoxy resin, and (4) changing thermal hysteresis for a hot-pressing process. These countermeasures have produced results to some extent.
In actuality, however, the above countermeasures have failed to completely correct warping or torsion of copper clad laminates. These problems are almost solved, when a double-sided copper clad laminate is coated with a copper foil of the same quality and same thickness on both sides, but not solved completely when a foil for one side differs in thickness from that for the other side.
Recently, the double-sided copper clad laminate is frequently coated with copper foils of different thicknesses, as production methods of printed-wiring boards are diversified. Therefore, the market is strongly demanding the effective means for solving these problems.